This is a long-shot, but maybe worth checking.....

As I recall, there are a couple sensors where the wire connections can inadvertently be swapped. I don't recall if it was the cold start valve and the coolant temp, or the cold start and the knock sensor, or.....?

But anyway, you might check for the following wire colors. (Make sure you're looking at the rear most coolant temp sensor, not the one for the gauge.):

Idle control:
Red/Black
Green (or possibly Blue/Yellow)

Coolant Temp:
Gray/White
Red/Black

Knock Sensor:
Black
Green

Cold Start Valve:
Gray/Black
Blue/Green

I'm thinking that if the wrong leads somehow got connected to the knock sensor it could severely retard the spark.

Also, if you find one of the wires running to the cold start valve has been snipped, don't be alarmed as that was Volvo's cure for cold start flooding on the Regina cars. That mod will generate a permanent Cold Start Valve fault code - completely expected.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)